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Article

Optimizing the Incorporation Modes of TiO2 in TiO2-Al2O3 Composites for Enhancing Hydrodesulfurization Performance of Corresponding NiMoP-Supported Catalysts

by
Ranran Hou
,
Qinghe Yang
,
Shuangqin Zeng
,
Jun Bao
,
Hong Nie
*,
Chuangchuang Yang
,
Yanzi Jia
,
Anpeng Hu
and
Qiaoling Dai
SINOPEC Research Institute of Petroleum Processing Co., Ltd., 18 Xue Yuan Road, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(5), 287; https://doi.org/10.3390/catal14050287
Submission received: 28 March 2024 / Revised: 18 April 2024 / Accepted: 22 April 2024 / Published: 24 April 2024
(This article belongs to the Section Catalytic Materials)
Browse Figures
Versions Notes

Abstract

:
TiO2-Al2O3 supports with different incorporation methods of titania were synthesized via three methods: impregnation (TA-I), co-precipitation (TA-CP), and co-precipitation–hydrothermal treatment (TA-HT). And the NiMoP catalysts prepared on the corresponding supports were evaluated for hydrodesulfurization (HDS) reactions. The results demonstrated that the Ti atoms in TA-I are attached to alumina through hydroxyl groups, while the Ti atoms in TA-CP and TA-HT can be dispersed in the alumina skeleton. Variations in the incorporation modes of TiO2 affect the support properties, consequently influencing the nature of the active metal on the supports. The Ti atoms dispersed in the Al2O3 skeleton allow an increase in the basic hydroxyl groups. Meanwhile, TiO2 in TA-CP and TA-HT can absorb hydrogen molecules and be partially reduced. Furthermore, metal species supported on the TA-CP and TA-HT are more easily reduced and better dispersed. For the NiMoP catalysts prepared with TA-CP and TA-HT, the Ti element promotes the sulfidation degree of Mo, besides shortening the average (Ni)MoS2 slab. The catalysts prepared with TA-CP exhibited superior activity for 4,6-DMDBT hydrodesulfurization. This can be ascribed not only to the relatively high sulfidation degree of Mo and proportion of the NiMoS active phase but also to the well-dispersed (Ni)MoS2 slabs. Moreover, the Ti4+ ions dispersed in the Al2O3 skeleton can be partially reduced to act as electron donors, enhancing the metallic character of the S layers in MoS2, which facilitates the improvement of the hydrogenation desulfurization activity.

1. Introduction

With increasingly stringent environmental standards, the deep removal of sulfur components in crude oil poses a major challenge to the refining industry [1,2,3,4]. Hydrodesulfurization (HDS) is considered to be the most efficient process to remove these sulfur compounds [5], and the design and investigation of catalysts is a critical factor in improving HDS catalytic performance. Currently, the most widely utilized HDS catalysts are bulk catalysts and supported catalysts. Although bulk catalysts exhibit superior performance, their high metal content and high price restrict their application. From this point of view, supported HDS catalysts with relatively low metal content and better textural characteristics can be a more attractive option. Supported catalyst design and optimization involve various parameters, such as the pore structure of the support and the morphology of the active phase [6,7,8,9]. The superior pore structure enhances the diffusion of reactant and product molecules and facilitates the dispersion of active metals [10,11]. In addition, the support surface properties determine the metal–support interaction, which further contributes to the morphology of the active metal in sulfided hydrogenation catalysts [12,13,14].
γ-Al2O3 is frequently applied as an industrial hydrogenation catalyst support material for its low cost, relatively superior pore structure, excellent stability, and easy formation. Nonetheless, environmental regulations require deep removal of organosulfur compounds such as 4,6-dimethylbenzothiophene (4,6-DMDBT), and the steric hindrance caused by methyl groups makes those compounds hard to remove; in some cases, it is difficult to meet the requirement for HDS catalyst capacity using pure γ-Al2O3 as a support [7,8]. Among other candidate support materials, researchers found that TiO2-supported catalysts show higher HDS activity than γ-Al2O3-supported catalysts due to the presence of “chemical ligand effects” in titania [15,16,17]. However, the low thermal stability and low surface area of TiO2 hinder its application as an industrial catalyst support. Thus, considerable efforts have been made to prepare high-performance composite support materials containing TiO2, especially TiO2-Al2O3 composites. Santes et al. [18] prepared three TiO2-Al2O3 composites by different methods: impregnation, co-precipitation, and the sol–gel method. The TiO2-Al2O3 composite prepared by the sol–gel method exhibited better pore distribution and higher pore volume, while the composite prepared by the co-precipitation approach had the largest surface area. For the TiO2-Al2O3 composites prepared via the impregnation method, XRD patterns evidence the formation of anatase crystallites and verify a poor dispersion of TiO2. Gasoil hydrotreating evaluations showed that the activity of the catalysts was improved by introducing TiO2 into the γ-Al2O3 support. Huang et al. [19] prepared different Ti/Al molar ratios of TiO2-Al2O3 binary oxides through a pre-hydrolysis co-precipitation procedure. As verified by the XRD patterns, there is almost no complete crystal phase of Al2O3 or TiO2 in the supports with a Ti/Al molar ratio of 1 or 2. TiO2-Al2O3 supports presented moderate metal–support interaction (MSI) and higher sulfidation degree of active metal. In addition, the quantity of coordinatively unsaturated sites (CUSs) increased with the introduction of Ti elements. The above-mentioned characteristics are conducive to improving the catalytic performance in the heavy oil hydrotreating process. Zhang et al. [20] synthesized highly ordered mesoporous TiO2-Al2O3 composite oxides with different Ti/Al molar ratios using a facile self-assembly method triggered by evaporation. The highly ordered TiO2-Al2O3 composites possess a highly specific area and narrow pore size distribution. It was demonstrated that the introduction of TiO2 could weaken the MSI and favor the formation of the “Type Ⅱ” Ni-Mo-S active phase. Furthermore, the NiMo catalyst with a Ti/Al molar ratio of 0.4 showed the highest 4,6-DMDBT HDS performance. Ferdous et al. [21] investigated the physicochemical properties of a series of NiMo catalysts supported on TiO2-Al2O3. The introduction of titania contributed to improving the dispersion degree of active metal and facilitating the formation of poly-molybdenum oxide on the catalysts. As a result, titania-modified Al2O3 could facilitate the HDS and hydrodenitrogenation (HDN) activities of catalysts. The study verified that introducing TiO2 to Al2O3 can enhance the sulfidation degree of the active metal and the catalytic performance. Nevertheless, the present research has focused on evaluating the effects of introducing TiO2 and variation in TiO2 content on catalytic performance. The characteristics of TiO2-Al2O3 synthesized by different methods varied, and the connection pattern of Ti and Al atoms is the main reason for this phenomenon, which has not been studied in detail. Therefore, the effect of incorporation modes and existence states of titania in TiO2-Al2O3 on the properties of the catalytic materials and NiMoP catalyst performance was further investigated.
In this study, a series of TiO2-Al2O3 composites were synthesized via co-precipitation, impregnation, and co-precipitation–hydrothermal methods. The pore structures of the four investigated supports are generally homogeneous, and appropriate characterization methods were used to determine the crystallinity and the variation in hydroxyl group intensity of samples. The reduction and the dispersion degree of Ti atoms were evaluated. The corresponding NiMoP-supported catalysts were characterized to investigate the coordination states of NiMo precursors and the variations of their interaction with TiO2-Al2O3 supports. After sulfidation, the active phase morphologies and the covalent states of sulfided catalysts were examined. Finally, 4,6-dimethylbenzothiophene (4,6-DMDBT) was selected as a probe molecule to explore the influence of the incorporation modes of TiO2 on the hydrodesulfurization performances of the corresponding catalysts.

2. Results and Discussion

2.1. Properties of the Synthesized Supports

The XRD patterns of the TiO2-Al2O3 together with the Al2O3 supports were recorded, and the results are shown in Figure 1. As Figure 1 shows, all samples exhibited the diffraction peaks of γ-Al2O3 (2θ of about 36°, 45°, and 67°) (JCPDS card No. 10-0425). In addition, the diffraction peaks at about 25°, 48°, 54°, and 55° for the TA-I and TA-HT samples are identified as anatase (JCPDS card No. 21-1272). For TA-I, when the amount of the titania exceeded the largest capacity of the monolayer distribution on Al2O3, TiO2 existed in the form of anatase by aggregation. The presence of XRD patterns of anatase indicates poor dispersion of TiO2 in TiO2-Al2O3 synthesized via the impregnation method, which is consistent with the results in the literature [18]. As shown in Figure 1, TA-CP did not show any anatase diffraction peaks; this result implied that the Ti atoms entered and were well dispersed in the Al2O3 skeleton during the co-precipitation process. However, after hydrothermal treatment, the Ti atoms dispersed in Al2O3 aggregated through hydroxyl linkage and showed anatase diffraction peaks. The XRD results for all TiO2-containing supports demonstrated that the introduction of TiO2 affects the intensity of the diffraction peaks of γ-Al2O3, indicating that the alumina particle size and crystallinity were changed.
Figure 2 displays the pore diameter distributions of synthesized TiO2-Al2O3 and Al2O3 supports, and Table 1 shows a summary of the pore structures of synthesized samples. The specific surface area, pore volume, and average pore size of TA-I and TA-CP supports were slightly reduced compared to those of Al2O3 support. The aggregation of TiO2 on the composite support affects the textural parameters [19]. In the case of TA-I, the TiO2 particles agglomerated on the surface of the Al2O3 support, which decreased the pore volume, surface area, and average pore diameter. Meanwhile, the pore volume and specific surface area of TA-HT were almost similar to those of the Al2O3 support. Moreover, for TA-HT, the average pore diameter was slightly larger than that of Al2O3 support.
Compared with TA-CP, the textural changes of TA-HT should be mainly attributed to the formation and growth of boehmite crystals during the hydrothermal treatment. As calculated by Scherrer’s formula [22], the grain size of the boehmite particles increased from 3.3 nm to 6.2 nm after 150 °C for 4 h hydrothermal treatment. With the particle growth during hydrothermal treatment, TA-HT exhibited a higher average pore diameter and narrower pore distribution than TA-CP.
Figure 3 shows the FTIR spectra of TiO2-Al2O3 support synthesized by various methods. The pure TiO2 support exhibits a broad band around 450 cm−1 to 680 cm−1, where the IR band at 495 cm−1 is assigned to the Ti-O-Ti bridge vibration of the anatase, and the bands between 550 and 653 cm−1 are attributed to the Ti-O vibrations [23]. The AlO4 vibrations are responsible for the bands 700 and 900 cm−1, while the AlO6 stretching is responsible for the band at roughly 600 cm−1 [24,25]. There was overlapping of the characteristic vibrational intervals of TiO2 and Al2O3 and a relatively small amount of titania, and no obvious Ti-O bonding vibrations were seen in the FTIR spectrum of supports. Characteristic vibrations of hetero metal–oxygen bonds of Ti-O-Al appear at 439 and 583 cm−1 [26]. Based on the spectral results, the intensity of the TA-I at 583 cm−1 was significantly higher than that of other samples, which indicates the presence of multiple Ti-O-Al bonds in TA-I. Therefore, when a TiO2-Al2O3 support is synthesized through the impregnation method, TiO2 and Al2O3 are preferentially connected through the surface hydroxyl groups.
Furthermore, Al2O3 and TA-I exhibited characteristic peaks at 1100 cm−1, which could be ascribed to an Al-O bond. On the contrary, both TA-CP and TA-HT exhibited almost no variation at 1100 cm−1, demonstrating that the presence of TiO2 affects the Al-O bond when TiO2-Al2O3 is synthesized by the co-precipitation method [27]. During the co-precipitation process, the Ti atom is proposed to be incorporated into the surface vacancy of Al2O3. According to the absence of any XRD characteristic peaks of TiO2 in TA-CP, it can be concluded that the dispersion of TiO2 in TA-CP is better with the same amount of TiO2 doping. This phenomenon is consistent with the embedding model which has been proved in many works in the literature [28,29,30].
Given that the nature and amount of surface hydroxyl groups determine the dispersion of the metal species [31], we used IR spectroscopy to characterize the distribution of hydroxyl groups in the prepared supports, as illustrated in Figure 4. Generally, basic, neutral, and acidic hydroxyl groups were identified with the bands located around 3772, 3726, and 3680 cm−1, respectively [32]. Furthermore, for TiO2-Al2O3 supports, the band centered around 3720 cm−1 can be regarded as the Ti-OH group [33].
The results in Figure 4 indicate that, compared with Al2O3 support, the number of total hydroxyl groups, especially the number of basic hydroxyl groups, in TA-I decreased, which is attributed to the TiO2 connected with Al2O3 through the surface hydroxyl groups. According to previous research, the formation of the basic hydroxyl group is mainly caused by defects in the configuration of Al2O3 crystals [34]. Considering the FT-IR results and the increase in basic hydroxyl groups of TA-CP and TA-HT in combination, it can be stated that Ti atoms entered the surface vacancy and affected the lattice of alumina during the co-precipitation.
Moreover, the blue shift of the TA-CP and TA-HT neutral hydroxyl spectrum was due to the influence of the Ti-OH. Comparing the intensity of the hydroxyl group band at 3720 cm−1 for TA-CP and TA-HT supports, it can be found that the relative intensity of Ti-OH decreased after hydrothermal treatment. Based on the literature [35,36], when deionized water was used as the solvent, a high concentration of hydroxyl radicals was generated during the hydrothermal treatment, and hydroxyl radicals might interact with the surface titanium atoms. When the hydrothermal treatment temperature is above 100 °C, hydrogen bonds between chemisorbed water molecules and the lattice oxygen or adjacent OH− groups recombine as dehydration occurs, transforming weaker hydrogen bonds into stronger bonds [37]. Combined with the decrease in the relative intensity of Ti-OH compared to TA-CP, this result reveals that the hydroxyl groups that interacted with Ti atoms recombined and formed Ti-O-Ti during the hydrothermal treatment.
The titanium element distribution in the TiO2-Al2O3 supports was verified using EPMA, and the results are shown in Figure 5. As Figure 5 manifests, the distribution of titanium elements in the alumina matrix is significantly influenced by the preparation methods. The dispersion degree of Ti elements in TiO2-Al2O3 supports is increased in the order of TA-I < TA-HT < TA-CP.
As shown in Figure 5a, when the TiO2-Al2O3 support was made by the impregnation method, Ti elements were more likely to aggregate on the alumina supports. Based on the previous characterization results, the TiO2 in TA-I was connected to the Al2O3 through hydroxyl groups, but the number of hydroxyl groups of the Al2O3 was limited. As a result, the Ti elements in TA-I will aggregate and form anatase. Meanwhile, the Ti elements in TA-CP and TA-HT were more uniformly dispersed in the alumina skeleton, as shown in Figure 5b,c. Furthermore, the red dots that appear in Figure 5c suggest that Ti elements in TA-HT exhibited linkage during the hydrothermal treatment.
To identify the properties of various oxide specie present on the TiO2-Al2O3 composites, H2 temperature-programmed reduction (H2-TPR) analysis was conducted, and the profiles are given in Figure 6. As can be observed in Figure 6, all synthesized supports show hydrogen reduction peaks between 450 and 700 °C, which can be partly attributed to the reduction of sulfate [38]. For TiO2-containing supports, the H2 consumption profile located at 430 °C is also associated with the reduction of anatase [39]. The low-temperature reduction peak temperatures of the synthesized support increase in the order of TiO2 < TA-I < Al2O3 < TA-CP < TA-HT. For TiO2-Al2O3 supports, the low-temperature reduction peaks were shifted towards higher temperatures; that phenomenon is associated with titania stabilization by interaction with the alumina [40]. Owing to the higher temperature of the low-temperature reduction peaks of TA-CP and TA-HT, we can conclude that the interaction between Al2O3 and TiO2 is slightly stronger in the supports synthesized by the co-precipitation method.
During the heating process, anatase will undergo crystalline transformation to rutile at about 750 °C, and the rutile phase will appear at a reduction peak at Tmax > 770 °C in the H2-TPR profiles [41]. Compared with TA-I, TA-CP and TA-HT showed a more pronounced reduction peak located at Tmax > 770 °C, indicating the presence of more reducible Ti species in these supports. Unlike the conventional profiles, when the reduction temperature was below 400 °C, the TCD signals of TA-CP and TA-HT both showed a decreasing trend. Considering this result in combination with the mechanism of H2-TPR, it was presumed that Ti atoms dispersed in the alumina skeleton could absorb hydrogen molecules at low temperatures.

2.2. MSI of the Synthesized Catalysts

To further examine the effect of anatase incorporation in alumina on MSI, H2-TPR characterization was employed, and the obtained profiles are exhibited in Figure 7. There are mainly three peaks: The peak located at around 300~400 °C is attributed to the reduction from Mo6+ to Mo4+, and the peak centered at approximately 500 °C is assigned to the reduction of the Mo6+ species that strongly interacted with the TiO2 to the lower coordinated species. The high-temperature reduction peak centered at around 800 °C is attributed to the reduction from Mo4+ to Mo0 [8]. Besides that, the higher temperature peak between the 542 °C and 812 °C interval of CAT-T is also partly attributed to the reduction of the support material itself [42,43].
The position of the low-temperature reduction peak of CAT-T shifted to a higher temperature relative to CAT-A, indicating that the reduction of Mo6+ loaded on the reducible titania to Mo4+ was more difficult. The low-temperature reduction peaks of CAT-CP and CAT-HT were shifted to lower temperatures, demonstrating that TA-CP and TA-HT supports were conducive to Mo6+ reduction compared to Al2O3 and TA-I. Following the support TPR results, TA-CP and TA-HT will absorb hydrogen at low temperatures (<400 °C), a trait that promotes Mo6+ reduction. On the contrary, TA-I exhibited the opposite effect on the Mo6+ reduction process. Given the above, when titania is connected with alumina through hydroxyl groups, e.g., CAT-I, the reduction of active metals supported on TA-I is more difficult. The TPR peak centered at around 800 °C of TiO2-containing catalysts, especially CAT-CP and CAT-HT, shifted to a higher temperature; this phenomenon demonstrates the SMSI, which is consistent with the relatively high intensity of the basic hydroxyl peaks [34] and the existence of Ti-OH of TA-CP and TA-HT supports. This SMSI also has been identified from the Raman results of Figure S1 in the Supplementary Materials. Moreover, CAT-CP and CAT-HT showed a shoulder peak of 400~600 °C, which can be attributed not only to the reduction of high coordinated Mo that interacted with TiO2 but also to the presence of the Ti4+ reduction.

2.3. Morphologies of the Sulfided Catalysts

Representative HRTEM micrographs for further examination of the effect of TiO2 modification on the morphology of the sulfided catalysts are displayed in Figure 8, and the statistical results are summarized in Table 2. The typical thread-like layers of (Ni)MoS2 slabs could be seen in all the HRTEM images. Table 2 indicates that the degree of (Ni)MoS2 slab stacking is not significantly affected by the introduction of TiO2. In contrast, the length of the (Ni)MoS2 slabs shortened following the incorporation of TiO2. The average slab length of (Ni)MoS2 increases in the following order: CAT-CP < CAT-HT < CAT-I < CAT-A. For TA-CP and TA-HT, both of which have more basic hydroxyl groups and Ti-OH, accordingly, the dispersion degree of Mo is higher, and the average (Ni)MoS2 slab length of sulfided CAT-CP and CAT-HT is shorter. The higher dispersion degree of active metals on titania-containing supports is consistent with the Raman results of Figure S1 in the Supplementary Materials.

2.4. Covalent States of the Sulfided Catalysts

X-ray photoelectron spectroscopy (XPS) was used to evaluate the effect of Ti species on the covalent states and sulfidation degrees of the active metals in the sulfided catalysts. Moreover, the deconvolution results according to the literature method [44,45,46] are shown in Figure 9a, and the calculated results are shown in Table 3. As observed in Figure 9a, the area of Mo5+ and Mo6+ peaks of CAT-CP and CAT-HT decreased notably, indicating that the sulfidation degree of Mo (Mosulfidation) was enhanced compared with CAT-A. On the contrary, the Mo5+ and Mo6+ peak areas of CAT-I increased relative to CAT-A, which can be explained by the results of H2-TPR, where the low-temperature reduction peaks of CAT-CP and CAT-HT were shifted to a lower temperature. This feature makes it easier to reduce Mo6+ to Mo4+ during the sulfidation process. The reduction of Mo6+ to Mo4+ is difficult in CAT-I, and the sulfation degree of Mo is relatively lower. The calculated results are summarized in Table 3, and the Mosulfidation increases according to the order of CAT-I < CAT-A < CAT-HT < CAT-CP.
The proportions of the NiMoS phase in the investigated catalysts were calculated based on Figure 9b, and the results are summarized in Table 3. Table 3 indicates that the proportion of the NiMoS phase in titania-containing catalysts is relatively higher. This phenomenon can be ascribed to the fact that the introduction of TiO2 into Al2O3 prevents Ni from entering the alumina vacancies. Compared with CAT-I, the promotion of Ni is relatively weak on CAT-CP and CAT-HT. According to the available literature [47], MoS2 supported on titania demonstrates mainly Mo edges, and this structure with fewer defective sites does not favor Ni promotion. Moreover, in comparison with CAT-CP and CAT-HT, after hydrothermal treatment, the Ti elements dispersed in the alumina skeleton showed anatase characteristic peaks through linkage and aggregation, and the proportion of the NiMoS phase was further reduced.

2.5. Catalytic Performance Evaluation

In accordance with the above characterization results and analysis, it can be determined that the different forms of Ti elements in alumina will have different effects on the properties of the catalysts. To further analyze the influence of Ti elements on catalytic performance, 4,6-DMDBT was selected as the probe molecule, and the HDS test results over the sulfided catalysts at temperatures ranging from 260 °C to 300 °C are displayed in Figure 10. As Figure 10 shows, the total HDS activity (Atotal) over the sulfided catalysts increased with increasing reaction temperatures. In addition, the Atotal increases in a sequence of CAT-I < CAT-A < CAT-HT < CAT-CP at all investigated reaction temperatures.
According to previous research [20,48], the HDS of 4,6-DMDBT mainly proceeds through two parallel reaction pathways, as illustrated in Scheme 1. As shown in Scheme 1, 3,3′-dimethylbiphenyl (DM-BP) is formed through a direct desulfurization pathway (DDS), while the DM-BCH and DM-BCH are formed through a hydrogenation desulfurization. The steric hindrance posed by the presence of two methyl groups located at the 4 and 6 positions determines that the HYD route is the predominant reaction pathway during the 4,6-DMDBT HDS reaction.
To further investigate the HDS selectivity of 4,6-DMDBT, the AHYD and ADDS at reaction temperature 280 °C were calculated by Equations (2) and (3), and the results are noted in Table 4. Table 4 indicates that the ADDS increased slightly after the addition of titania. In the meantime, the AHYD of CAT-I decreased, whereas the AHYD of CAT-CP and CAT-HT increased. For CAT-I and CAT-HT, the characteristic peaks of anatase have been detected; under the same ADDS, the AHYD of CAT-HT is relatively higher. Correspondingly, for CAT-CP, in which Ti is the most dispersed in the alumina skeleton, the AHYD is the highest.
Combined with the above characterization analysis of TiO2-Al2O3 supports obtained by various preparation methods and corresponding synthesized catalysts, the following explanation is proposed for the differences in HDS results. When TiO2-Al2O3 supports were synthesized by the co-precipitation method, Ti atoms were proposed to be incorporated into the surface vacancies of Al2O3. In this way, the reduction of Mo6+ loaded on the supports is promoted, which is conducive to increasing the sulfidation degree of Mo, e.g., CAT-CP (70.08%) and CAT-HT (68.89%). Moreover, with the relatively similar Mosulfidation of CAT-CP and CAT-HT, the higher HDS activity of CAT-CP can be ascribed to the higher proportion of the NiMoS phase. Under hydrothermal treatment conditions, Ti atoms dispersed in alumina gradually connected and aggregated through Ti-OH, and the promotion of Ni on MoS2 slabs was further weakened with the aggregation of TiO2. But despite CAT-I having the highest NiMoS proportion of all catalysts, the activity is still low due to its lower Mosulfidation.
On the other hand, the intensity of basic hydroxyl groups on TA-CP and TA-HT supports are relatively higher, along with the appearance of a more pronounced characteristic peak of Ti-OH; these changes result in strong metal support interaction and increased Mo dispersion. This property makes the average (Ni)MoS2 slab length of sulfided CAT-CP and CAT-HT relatively small; in particular, the (Ni)MoS2 slab length of CAT-CP is significantly reduced. According to a previous literature study [49], for the same Mo atom loading, the larger (Ni)MoS2 slabs imply fewer potential active sites. Therefore, the HDS activity of CAT-CP with a significantly short average (Ni)MoS2 slab length (3.03 nm) was higher.
Furthermore, when Ti atoms are combined with Al2O3 via incorporation into surface vacancies and well dispersed in the Al2O3, Ti4+ ions in TA-CP and TA-HT can be partially reduced to Ti3+ ions under reduction conditions. The Ti3+ ions depict the existence of electronic density in both titania and molybdenum sulfide conduction bands. The delivery of electronic density between the titania and molybdenum sulfide causes the greater metallic character of the S layer in MoS2 particles [47,50,51]. In the meantime, the greater metallic character of the MoS2 presents higher hydrogenating capability. As a result, compared to the HYD activity of CAT-I (0.42), the CAT-CP (0.48) and CAT-HT (0.46) with reducible Ti species present higher HYD activity.

3. Materials and Methods

3.1. Materials

Raw materials: (1) aluminum sulfate (Al2(SO4)3) solution, Al2O3: 104 g·L−1; (2) sodium aluminate (NaAlO2) solution, Al2O3: 220 g·L−1, Na2O: 227 g·L−1; (3) titanium oxysulfate–sulfuric acid hydrate, Innochem (Beijing, China), synthesis grade; (4) titanium isopropoxide, Innochem (Beijing, China), 98%; (5) TiO2, Saint-Gobain Norpro (Shanghai, China), ST61120; (6) molybdenum oxide, alkaline nickel carbonate, and phosphoric acid, Sinopharm Chemical Reagent Co., Ltd. (Beijing, China).

3.2. Synthesis of TiO2-Al2O3 Composite Oxide Supports

Three different kinds of TiO2-Al2O3 (10:90 wt%) supports were synthesized. The first one, named TA-I, was synthesized by the impregnation method. An appropriate amount of titanium isopropoxide was added to n-propanol to configure an impregnation solution, and then the impregnation solution was mixed with the butterfly-shaped extruded Al2O3 supports (SINOPEC Research Institute of Petroleum Processing Co., Ltd., Beijing, China) with a diameter of 1.6 mm via the pore volume impregnation method. After the impregnation process, the modified samples were dried at 120 °C for 3 h and calcined at 550 °C for 3 h.
The other two TiO2-Al2O3 composites were prepared by co-precipitation or hydrothermal methods and then molded to obtain the supports. In the procedure of preparing TiO2-Al2O3 by co-precipitation, titanium oxysulfate–sulfuric acid hydrate was added to aluminum sulfate solution to form a homogeneous acidic solution with a concentration of 12.9 g·L−1 in terms of TiO2 and 57.2 g·L−1 in terms of Al2O3. Then, the acid solution and the sodium aluminate solution were added to the tank simultaneously at a certain flow rate under vigorous stirring, and the pH of the reaction solution was controlled between 6.2 and 6.5. At the end of the co-precipitation session, sodium carbonate solution was added to the mixture to reach a pH between 8.9 and 9.2. Finally, the solution mixture was heated at 90 °C with stirring and aged for 6 h. The slurry was subsequently filtered and washed, and the cake was divided into two parts. One part was directly spray dried followed by extrusion into butterfly-shaped particles with a diameter of 1.6 mm and then dried in air at 120 °C for 3 h and calcined at 550 °C for 3 h, and the obtained support was recorded as TA-CP. The other part of the cake was redispersed in water and heated at 150 °C while being stirred in a hydrothermal autoclave. The resulting gel was washed with deionized water and dried and then extruded to obtain butterfly-shaped particles of 1.6 mm in diameter. Finally, the support, named TA-HT, was obtained by drying at 120 °C for 3 h and calcined at 550 °C for 3 h.

3.3. Preparation of NiMoP-Supported Catalysts

An incipient wetness co-impregnation method was used to prepare the corresponding NiMoP catalysts. A mixture of a solution containing molybdenum oxide, alkaline nickel carbonate, and phosphoric acid was used as the impregnation solution. The wet catalysts were first dried at 120 °C for 3 h and then calcined at 400 °C for 3 h. The catalysts possessed the same loading of 14.5 wt% MoO3, 3.5 wt% NiO, and 3.0 wt% P2O5. The obtained catalysts were denoted as CAT-I, CAT-CP, and CAT-HT, respectively. When the support was pure Al2O3 or TiO2, the corresponding catalyst was denoted as CAT-A or CAT-T.

3.4. Characterization of the Materials

The crystalline structure of the prepared samples was determined by wide-angle powder X-ray diffraction (XRD). The characterization was carried out on a Philips (Andover, MA, USA) XPERT advanced powder diffractometer with Cu Kα radiation (40 kV, 30 mA).
The pore structure of each support was characterized through nitrogen adsorption–desorption measurements at −196 °C on a Micromeritics (Norcross, GA, USA) ASAP 2420 analyzer. The samples were outgassed at 350 °C for 10 h while under vacuum before measurement.
The formation of interaction bands and the presence of hydroxyl groups in the range of 4000–400 cm−1 were assessed using Fourier-transform infrared spectroscopy (FT-IR) on a Nicolet (Green Bay, WI, USA) 870 Fourier-transform infrared spectrometer.
The elemental distribution of titanium and aluminum in the TiO2-Al2O3 composites was examined by a JXA-8230 electron probe microanalyzer (EPMA) (Tokyo, Japan). The analyses were executed at a 15 kV accelerating voltage.
H2 temperature-programmed reduction (H2-TPR) of synthesized samples was performed to characterize the MSI on an AutoChemⅡ2920 apparatus (Norcross, GA, USA). The flow rate was 10% H2/Ar (by volume) at 50 cm3·min−1, and the temperature was raised to 950 °C at a rate of 10 °C·min−1, with a cold trap to condense the water vapor.
The Fourier-transformed Raman (FT-Raman) spectra for these catalysts were noted on a LabRAM (St. Petersburg, FL, USA) HR UV-NIR Raman apparatus equipped with a He-Cd laser (325 nm) to determine the NiMo precursor coordination states on the binary oxide surface.
High-resolution transmission electron microscopy (HRTEM) images of sulfided catalysts were recorded on a Tecnai G2 F20 S-TWIN microscope (Hillsboro, OR, USA). Before the characterization, catalyst samples with a mesh size of 40–60 were sulfided utilizing the sulfidation method in a fixed-bed micro-reactor with 5 wt% CS2 cyclohexane solution. The sulfidation procedure was performed at a temperature of 230 °C for one hour, and then at 360 °C for two hours, with a total pressure of 4.0 MPa, a liquid flow rate of 0.4 mL·min−1, and a H2 flow rate of 365 mL·min−1. The sulfided catalysts were carefully unloaded and then put into cyclohexane to prevent oxidation. Each sample was captured in at least 20 micrographs, The (Ni)MoS slab morphology data for each sample were obtained by counting at least 800 slabs. The morphological details of the (Ni)MoS slab were statistically calculated as described in Section S1 in the Supplementary Materials.
X-ray photoelectron spectroscopy (XPS) was employed to determine the coordination states of the active metals on the sulfided catalysts. The XPS analysis was performed on a Thermo Fischer (Waltham, MA, USA)-VG ESCALAB 250 spectrometer using Al Kα radiation.

3.5. Catalytic Performance Assessment

The catalytic performance of the synthesized catalysts was determined using 4,6-DMDBT as the reactant and assessed on a continuous-flow fixed-bed micro-reactor with an inner diameter of 8 mm. And the sulfidation procedure and catalytic performance assessment method were described in the HRTEM characterization.
Then, the 4,6-DMDBT (0.45 wt% in decane) HDS reaction was performed at temperatures ranging from 260 °C to 300 °C. Decahydronaphthalene was added as an internal standard. The other test conditions were as follows: the liquid flow rate was 0.2 mL·min−1, the total pressure was 4.0 MPa, and the H2/oil volume ratio was 500. Following stabilization for 2 h, the liquid products were gathered and subjected to an offline analysis using an Agilent (Santa Clara, CA, USA) 7890A Gas Chromatograph equipped with a 30 m capillary column of HP-15 methyl siloxane and a flame ionization detector (FID).
There are two main mechanisms for the HDS reaction; one involves the substitution of a hydrogen atom for a sulfur atom in the hydrocarbon structure, defined as the direct desulfurization pathway (DDS), and the other includes the hydrogenation of the benzene ring followed by removal of the S atom, which is defined as hydrogenation desulfurization (HYD) [48]. As is verified, 4,6-THDMDBT, 4,6-HHDMDBT, 4,6-DM-PHDBT, 3,3′-DM-CHB, and 3,3′-DM-BCH are obtained by the HYD pathway, while 3,3′-DMBP is obtained by the DDS pathway [14]. The activity of 4,6-DMDBT HDS reaction over the prepared catalysts was calculated in the range of 260–300 °C according to the following equations [52]:
A T = F 0 × x m
A D D S = A T × S D D S
A H Y D = A T × S H Y D
In Equations (1)–(3), F0 stands for the molar flow rate of 4,6-DMDBT in mol·h−1, x stands for the 4,6-DMDBT conversion, and m stands for the catalyst mass in kg. SDDS is the selectivity of the DDS route, and SHYD is the selectivity of the HYD route.

4. Conclusions

Titania–alumina composite oxides with a TiO2 mass ratio of 10% were prepared by an impregnation method, co-precipitation method, and co-precipitation–hydrothermal treatment method. The effect of incorporation modes on the properties of TiO2-Al2O3 supports and the HDS performance of NiMoP catalysts was investigated. Based on the characterization of supports and corresponding catalysts, we concluded that TiO2 and Al2O3 are connected via surface hydroxyl groups in TA-I, whereas Ti atoms can be dispersed in the alumina skeleton and affect the nature of Al-O in TA-CP and TA-HT. In addition, Ti atoms dispersed in the alumina skeleton can promote the reduction of Mo6+ to Mo4+ and further increase the sulfidation degree of Mo. For TA-CP and TA-HT, the increase in the intensity of basic hydroxyl and titanium hydroxyl groups is conducive to the dispersion of the active metal on supports and shortens the average Ni-promoted MoS2 slab length. Depending on the higher dispersion degree of Ti atoms in TA-CP, the titanium hydroxyl group intensity is higher; consequently, the average length of (Ni)MoS2 slabs in the sulfided catalyst is shorter. Moreover, TiO2 in CAT-CP and CAT-HT can be partially reduced to Ti3+, which makes MoS2 more metallic due to its unique electronic effect.
Afterward, the HDS reaction of 4,6-DMDBT assessed the corresponding catalysts’ activity. Optimal HDS activity of 4,6-DMDBT was obtained over CAT-CP, which was mainly attributed to the well-dispersed Ti atoms in the alumina skeleton not only improving the dispersion of the active metal but also increasing the Mo sulfidation degree and the proportion of NiMoS. In addition, the reducible TiO2 also changed the morphology of MoS2 slabs and enhanced HYD activity. This study presents ideas for synthesizing TiO2-Al2O3 supports with high HDS performance.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14050287/s1, Section S1 Statistical calculation methods of HRTEM, Figure S1: Raman spectra of the synthesized catalysts. Figure S2: Wide-angle XRD patterns of the synthesized catalysts. Refs. [51,53,54,55,56,57,58,59,60,61] are cited in Supplementary Materials.

Author Contributions

Conceptualization, R.H. and S.Z.; formal analysis, R.H. and S.Z.; investigation, R.H., S.Z. and J.B.; writing—original draft, R.H.; writing—review and editing, R.H., Q.Y., S.Z., J.B., H.N., C.Y., Y.J., A.H. and Q.D.; supervision, Q.Y. and H.N. All authors have read and agreed to the published version of the manuscript.

Funding

This work has been financially supported by a research grant from the National Key R&D Program of China (No. 2021YFA1501204) and the Project of SINOPEC (No. 123018).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare that this study received funding from SINOPEC Research Institute of Petroleum Processing Co., Ltd. The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

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Figure 1. Wide-angle XRD results of the supports.
Figure 1. Wide-angle XRD results of the supports.
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Figure 2. Pore size distribution of the supports.
Figure 2. Pore size distribution of the supports.
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Figure 3. Skeletal FTIR spectra of TiO2-Al2O3 support with various preparation methods.
Figure 3. Skeletal FTIR spectra of TiO2-Al2O3 support with various preparation methods.
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Figure 4. IR bands for synthesized supports in the 3400–3800 cm−1 spectra region.
Figure 4. IR bands for synthesized supports in the 3400–3800 cm−1 spectra region.
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Figure 5. EPMA results for Ti elements in the TiO2-Al2O3 supports: (a) TA-I, (b) TA-CP, (c) TA-HT.
Figure 5. EPMA results for Ti elements in the TiO2-Al2O3 supports: (a) TA-I, (b) TA-CP, (c) TA-HT.
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Figure 6. H2-TPR profiles of the synthesized supports.
Figure 6. H2-TPR profiles of the synthesized supports.
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Figure 7. H2-TPR profiles of the synthesized catalysts.
Figure 7. H2-TPR profiles of the synthesized catalysts.
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Figure 8. Representative HRTEM images of the sulfided catalysts: (a) CAT-A, (b) CAT-I, (c) CAT-CP, (d) CAT-HT.
Figure 8. Representative HRTEM images of the sulfided catalysts: (a) CAT-A, (b) CAT-I, (c) CAT-CP, (d) CAT-HT.
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Figure 9. Deconvolution results for XPS of the sulfided catalysts.
Figure 9. Deconvolution results for XPS of the sulfided catalysts.
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Figure 10. HDS of 4,6-DMDBT results of the synthesized catalysts.
Figure 10. HDS of 4,6-DMDBT results of the synthesized catalysts.
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Scheme 1. The HDS reaction network for 4,6-DMDBT over synthesized catalysts.
Scheme 1. The HDS reaction network for 4,6-DMDBT over synthesized catalysts.
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Table 1. Textural properties of the supports.
Table 1. Textural properties of the supports.
SampleSBET, m2·g−1Vp, cm3·g−1Dave, nmAl2O3, wt%TiO2, wt%
Al2O32400.688.471000
TA-I2200.588.1688.9611.04
TA-CP2240.618.0289.3110.69
TA-HT2400.669.3989.2410.76
Table 2. Statistical details for average slab length, average stacking number, and DMo of (Ni) MoS2.
Table 2. Statistical details for average slab length, average stacking number, and DMo of (Ni) MoS2.
CatalystsCAT-ACAT-ICAT-CPCAT-HT
Average slab length, nm3.883.563.033.42
Average stacking number1.61.61.41.7
DMo0.260.270.330.29
Table 3. XPS calculated results of sulfided catalysts.
Table 3. XPS calculated results of sulfided catalysts.
SamplesSulfidation Degree of Mo, %Proportion of NiMoS, %
CAT-A65.6832.46
CAT-I62.7539.77
CAT-CP70.0838.92
CAT-HT68.8934.39
Table 4. Selectivity of 4,6-DMDBT HDS reaction over the sulfided catalysts at 280 °C.
Table 4. Selectivity of 4,6-DMDBT HDS reaction over the sulfided catalysts at 280 °C.
CatalystsCAT-ACAT-ICAT-CPCAT-HT
AHYD (mol·kg−1·h−1)0.450.420.480.46
ADDS (mol·kg−1·h−1)0.050.060.060.06
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Hou, R.; Yang, Q.; Zeng, S.; Bao, J.; Nie, H.; Yang, C.; Jia, Y.; Hu, A.; Dai, Q. Optimizing the Incorporation Modes of TiO2 in TiO2-Al2O3 Composites for Enhancing Hydrodesulfurization Performance of Corresponding NiMoP-Supported Catalysts. Catalysts 2024, 14, 287. https://doi.org/10.3390/catal14050287

AMA Style

Hou R, Yang Q, Zeng S, Bao J, Nie H, Yang C, Jia Y, Hu A, Dai Q. Optimizing the Incorporation Modes of TiO2 in TiO2-Al2O3 Composites for Enhancing Hydrodesulfurization Performance of Corresponding NiMoP-Supported Catalysts. Catalysts. 2024; 14(5):287. https://doi.org/10.3390/catal14050287

Chicago/Turabian Style

Hou, Ranran, Qinghe Yang, Shuangqin Zeng, Jun Bao, Hong Nie, Chuangchuang Yang, Yanzi Jia, Anpeng Hu, and Qiaoling Dai. 2024. "Optimizing the Incorporation Modes of TiO2 in TiO2-Al2O3 Composites for Enhancing Hydrodesulfurization Performance of Corresponding NiMoP-Supported Catalysts" Catalysts 14, no. 5: 287. https://doi.org/10.3390/catal14050287

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